Example Lesson:

Buoyancygravity

buoyancy

Why some objects float

Preliminary Concepts

Density = mass / volume -or- ρ = m / V

For example a rock has more mass and thus weighs more than a dinner roll of equal size.

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Which object do you think would be most likely to float in water, the stone or the dinner roll?

This is because the dinner roll is less dense.

Archimedes

According to legend…King Hiero II gave a goldsmith an amount of pure gold to construct a crown. After the crown was delivered the king became suspicious that the craftsman had stolen some of the gold and mixed the rest with silver. He asked Archimedes to determine if this was true without melting the crown.

Archimedes

While taking a bath, Archimedes noticed that the level of the water in the tub rose as he got in, and realized that this effect could be used to determine the volume of the crown. The submerged crown would displace an amount of water equal to its own volume. By dividing the mass of the crown by the volume of water displaced, the density of the crown could be obtained. This density would be lower than that of gold if cheaper and less dense metals had been added.

Archimedes

Archimedes then took to the streets naked, so excited by his discovery that he had forgotten to dress, crying "Eureka!" (meaning "I have found it!"). The test was conducted successfully, proving that silver had indeed been mixed in.

Archimedes Principle

A body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces

Downward force equal to the weightof the object

Upward force equal to the weightof the displaced fluid

=

Buoyancy

+An object that is less dense than water will float

- An object that is less dense than water will float

= An object that is just as dense as water will float

Positively Buoyant

Neutrally Buoyant

Negatively Buoyant

Buoyancy and ROVs

• Typically ROVs used by companies and universities are made to be slightly positively buoyant–Why might this be?

• This is so the ROV will float to the surface if it is damaged or communication is broken

Buoyancy and Your ROV

• Should you make your own ROV positively, negatively, or neutrally buoyant?

• What are the advantages and disadvantages of each?

• In this setting it is typically best to make an ROV neutrally buoyant

How to Change Buoyancy

To increase buoyancy add something that is very positively buoyant like foam

+ Floats

To decrease buoyancy add something that is very negatively buoyant like metal

- Ballast

Why Might You Want Both?

How to Actively Change Buoyancy

Rises Sinks

Adding air to a balloon onthe ROV makes it rise

Adding water to a tank onthe ROV makes it sink

Buoyancy Activity

• How could we determine the mass of an object without using a scale?

• Hints: – The density of water is 1g / mL– This will not work if the object sinks in

water– (Archimedes Principle applies to floating

objects)

Buoyancy Activity

• See how much water the object displaces

• Use the density equation to solve for mass

– ρ = m / V (remember ρ stands for density)– (1g / mL) = (mass) / ( volume measured in mL)– (1 g / mL) * ( # mL) = the object’s mass

• Check your answer using a scale